We have a collection with about 750k document (size - ~430 MB, storage size - ~170 MB).

When we tried to build an index with seven keys on that collection, the MongoDB server got very slow for the index build time. The queries and update operations got slowed down during the time the index was built.

We first encountered this problem after upgrading MongoDB on our servers up to 4.0.12 and was hoping that the new optimized build process.

While being on MongoDB 4.0.12, we would restart one of the secondary replicas and make that replica the primary one. That would completely resolve this issue.

However, after upgrading to 4.2.17 we're still experiencing this problem